Aug 12 2011
I’m ashamed to see that posts have been blank since May. I have been busy on another site related to Cr-48 Chromebook usage—but that’s not about this stuff.
In the past months, I’ve been grinding on the Marin Community Map, in particular working out the details of how park lands interact with the tidal reaches. This as graded into a representation of tidal lands, a pulling back of water polygons to lower-low water, and the start of harmonization with the San Francisco Estuary Institute’s Bay Area Aquatic Resource Inventory (BAARI).
I’ve spent hours dealing with topographic (elevation)-based definitions of shorelines such as were used in our model of San Francisco Bay Conservation and Development Commission (SF BCDC) jurisdiction. But as it turned out, all of our interesting marshes and tidal lands are tilted down toward the bay—go figure! So using guidelines for delineation that were very aptly documented by SFEI for BAARI, I started returning to the National Agricultural Imagery Program (NAIP) 1-meter, 4-band imagery of tidal lands for photointerpretation. In many cases, the wetlands were more appropriately mapped using the NAIP imagery than they were using terrain-derived contours. Features like tidal channels creep up to higher elevations while maintaining their widths, while contours tend to pinch out at some point and start going back down the other side of the tidal channel.
One of the by-products of all this attention is that I’ve split out the tidal lands around Marin in that span between lower-low water—which will be cartographically filled in with a blue polygon and bathymetry contours—and high water, where the public easement for beaches stops. For consistency, I’ve detailed out every little patch of space between these tidal ranges, all around the county, and only left out places that were plainly in private ownership, like a back yard with a dock. In keeping with BAARI criteria, I’ve used NAIP color infrared imagery to detail out polygon areas for vascular life forms (marshes) and the tidal channels and outboard mud flats around them.
Particularly good views of lower-low water were captured in NAIP 2005 imagery. Fair views of medium tide were found in NAIP 2009 imagery. A nice mix of low tide and improved quality near-infrared band data are in the NAIP 2010 images. In the end, I’m using NAIP 2005 to trace the outer limits of mud flats at lower-low water, and using NAIP 2010 to detail out the extent of marshes, because excellent red contrast makes it easy in that year’s data.
Also, we’ve had significant progress / closure on the ArcHydro generation of flow lines countywide for Marin and associated watersheds. As of now, we have got flow models for drainage networks below 1-hectare catchment in all of Lagunitas creek, and below 1-acre catchments elsewhere. These flow lines have been attributed with catchment area every 10 meters along their length, which has allowed us to provisionally classify them for perennial, intermittent, ephemeral, tidal, or impoounded flow. Also, we have attributed USGS NHD FCode feature codes for every segment as either a flow-specific creek, various storm drain pipes and ditches, or artificial paths through standing water. All of this stuff is being run on our 45cm topographic-bathymetric surface model, so all of the ArcHydro flow lines are running seamlessly through the tidal reaches and out into deep water. It’s been particularly interesting to see where soft sediments meet granite and other outcrops offshore, as flow lines go from largely parallel sheets to dendritic patterns even when they are underwater, using this technique.
The modeled flow lines can be found at this link.